Dispersive systems and spectral lines

The active part of the optical set-up begins with the entrance slit onto which is focused the light produced by the sample (the light source). The width of this slit is regulated by a mechanical system and cannot be less than a few micrometers because a minimum of light is required to enter, but its length can attain several centimetres. 

The source is thus transformed into an luminous object of linear profile of which the optical system of the instrument gives as many defined images — the lines — as there are different wavelengths in the source. Each line corresponds to an almost monochromatic image of the entrance slit of the spectrophotometer. A single element can generate more than 2000 lines. 

The lines are distributed in the focal plane as a succession of parallel and very narrow luminous segments. The width of these lines depends on several factors. 

At the beginning, the qualitative study of these spectra was made by visual comparison using spectrographs. Nowadays these have been replaced by spectrometers capable of resolving line interferences and problems linked to matrix effects. Each element, whether in neutral or ionized form, is responsible for a range of lines of variable intensities. The instrument is therefore programmed to both identify and to quantify the elements in the sample to be measured. Instruments are classified into two major groups. 

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